In current internal-combustion engines electromagnetic actuators have been used for operating engine valves, as disclosed, for example, in German Offenlegungsschrift (application published without examination) 195 18 056 to which corresponds U.S. Pat. No. 5,818,680.
An electromagnetically operated engine valve assembly for a piston-type internal-combustion engine essentially is formed of a valve body connected with a closing spring and an electromagnetic actuator which includes two electromagnets whose spaced pole faces are oriented towards one another and which includes an armature which may be reciprocated in the space between the pole faces of the two electromagnets. The armature is provided with a guide bar, one end of which is coupled with the valve stem and its other end is connected with an opening spring. Between the guide bar and the valve stem a valve clearance is provided.
A valve assembly of the above-outlined type constitutes an oscillatable spring/mass system whose total "mass" is formed essentially by the armature, its guide bar and the valve body whereas the "spring" is formed by the opening and closing springs. The assembly is conventionally designed in such a manner that both resetting springs are of identical configuration.
According to the basic principle of such engine valve assemblies, for reducing the electrical energy required for the operation, the natural oscillating capacity of the spring/mass system is utilized so that in principle the momentary capturing magnet and the holding magnet which retains the valve during the determined open and closed periods has to be supplied only with a limited current. The current intensity is such that the armature, as it passes its mid position, begins to be attracted by the capturing electromagnet, otherwise the kinetic energy of the total mass is utilized for a significant part of the motion.
As the armature approaches the pole face of the energized capturing electromagnet, the spring force of the oppositely acting resetting spring increases only linearly while in case of a constant current supply of the capturing electromagnet the magnetic forces derived therefrom and acting on the approaching armature increase exponentially. Since for ensuring a reliable capture of the armature, the electromagnetic force has to overcome the oppositely oriented increasing spring force, a number of methods have been developed for regulating the current supply. Such a regulation has the purpose of decreasing the current supply during the approach of the armature to the pole face of the capturing electromagnet. Such a reduction in the current supply reduces the magnetic force acting on the armature for ensuring a soft arrival of the armature on the pole face to thus avoid disadvantageous rebounding phenomena.
For regulating the current supply of the momentarily capturing electromagnet, it is necessary to detect the momentary position of the armature relative to the pole face and/or the velocity of the armature in the zone of approach. For this purpose methods have been developed which detect the feedback effects of the armature, as it moves through the magnetic field, on the supply current and voltage to derive therefrom the necessary signals for affecting the current supply.
In addition, regulating systems have been developed in which the momentary armature position and/or the momentary armature velocity is detected by sensors directly at the armature. Since for structural reasons alone the armature, together with its guide bar, and the valve body which is formed essentially of the valve head and valve stem cannot be made of a single piece, a clearance is necessarily present between the free end of the valve stem, on the one hand, and the adjoining end of the guide bar, on the other hand. The clearance, because of the varying temperature effects, changes during the operation of the internal-combustion engine.
In conventional hydraulic valve slack adjusting arrangements positioned between the guide bar and the valve stem an oil-chargeable cylinder has been used for bridging the valve clearance like a "rigid" intermediate layer, while compensating for any change in the valve clearance. Such valve slack adjusting systems are expensive to manufacture. Considering the possibilities of a fully variable electromagnetic valve drive, it is a desideratum to permit a valve clearance to make possible an avoidance of valve motion problems by suitably controlling the current supply. For such a control the detection of the armature motion and/or the armature velocity during the first phase of motion is of significance. Advantageously, signals are derived directly from the detection of the respective armature position and armature velocity.
By virtue of the presence of a valve clearance the oscillatable spring/mass system formed by the entire arrangement is divided into two partial systems which are timewise uncoupled. As a result, particularly at the beginning of the valve opening process, but also at the end of the valve closing process, the armature supported by the opening spring, together with its guide bar, may execute displacements relative to the valve body supported by the closing spring. As the guide bar of the armature strikes the valve stem, the spring/mass system formed by the two resetting springs and the masses of the armature and the valve is forced into a resonance oscillation which is quieted only when the armature arrives into contact with the opening magnet. The oscillating motion of the armature which is derived from the resonance oscillation and which is superposed on the armature travel makes even more difficult to guide the armature to the pole face of the capturing opening magnet with a possibly low impact velocity. Thus, despite a controlled guidance of the armature motion by a suitable control of the capturing current, disadvantageous rebound phenomena cannot be avoided.
For eliminating the above-noted rebound phenomena, according to U.S. Pat. No. 5,832,883 the armature is fixedly coupled with the valve body and a hydraulic dampening element is provided which is fixedly attached to the valve and by means of which the seating velocity of the valve is to be reduced. In this arrangement a valve clearance is not provided. This known system cannot be driven by utilizing the natural oscillation because of the continuously present dampening effect during the entire valve motion. The purpose of such dampening is to ensure a soft arrival of the valve into its valve seat. In such a construction the armature, in the seated position of the valve, may not come in contact with the pole face and further, for equalizing the braking effect of the dampening, element, the closing spring must be stronger than the opening spring. This results in a higher energy requirement, one reason being that the dampening element does not have a useful natural oscillation capability.
Disadvantageously, in seeking to reduce the total mass of the oscillatable system, only a reduction of the armature mass has been attempted on the grounds that a reduction of the valve body mass is very limited in view of the required valve material. After shutting off the holding current of the closing magnet, because of the high acceleration caused by the opening spring, the armature, as it strikes the valve stem, introduces a high energy boost into the partial system formed by the closing spring and the valve body even in case of a small valve clearance of 0.1 mm. As a result of such an energy boost, the armature executes, against the force of the opening spring, multiple oscillations which are superposed on the opening motion and further, at the same time, the valve body too, which moves in the opening direction, executes an oscillation superposed on the opening motion. Based on the multiple, out-of-phase, and oppositely oriented motions of the armature and the valve body, on both moving systems an oscillating motion is superposed which continues substantially beyond the mid position of the armature between the two pole faces.
If by means of a suitable sensor system the position of the armature and/or the armature velocity is detected directly at the armature, only a "washed-out" or "noisy" signal rather than an accurate signal can be obtained because of the oscillating motion of the armature.